Process and apparatus for electrostatically printing on hot substrate



Dec. 27, 1966 D F. ST. JOHN PROCESS AND APPARATUS FOR ELECTROSTATICALLYPRINTING ON HOT SUBSTRATE Filed 061;. 19, 1964 2; V2 24 24 J k j -l6 L"I A m V1 12 28 26 50 r r 521 X JO T154 7NVENTOIZ flbueugj F SrJ'or-nvUnited States Patent 3,294,017 PRGQESS AND APPARATUS FQlR ELECTRO-fiTATICALLY PRllNTlNG UN HOT SUBSTRATE Douglas P. t. .l'olrn, Toledo,@hio, assignor to Owens- Illinois, End, a corporation of ()hio FiiedGet. 19, 1964, Ser. No. 404,822 11 (Ilaims. (Cl. 101-114) This inventionrelates to electrical printing processes and apparatus, and moreparticularly to processes and apparatus especially adapted toelectrically transfer printing powder particles from a supply throughimage defining apertures in a stencil screen directly to a heatedsurface, as, for example, to the surface of a glass article still heatedto a relatively high temperature by residual heat from the articleforming process.

The present invention is especially concerned with problems encounteredin the so-called electrostatic printing or decorating of glass articles.In the formation of glass. articles such as bottles, for example, thearticles are produced in a forming machine and are discharged from theforming machine at a relatively high temperature of the order of 1000"to 1200 F. In conventional practice, the articles are then cooled andconducted to an annealing lehr and reheated to anneal the article. Mostprevious processes for decorating such articles have required that thearticle be cooled to a relatively low temperature before it was possibleto apply a decoration. In some cases, it was desirable to heat thedecoratin material after application to the article which suggested theapplication of the decoration between the forming machine and theannealing lehr. However, this was frequently found to be impractical incases where the article had to be cooled to a relatively low temperaturebefore the decorating process could be performed. Desirably, coolingbeyond a predetermined temperature between the forming machine andannealing lehr should be kept to a minimum because otherwise the articlemust stay in the lehr for a longer time before it reaches the annealingtemperature.

Experience has shown that the so-called electrostatic printing processis peculiarly well adapted to the decorating of glass articles duringthe period of transit of the article from the forming machine to theannealing lehr. As set forth in more detail in a copending applicationof Kenneth G. Lusher, Serial No. 242,229, filed December 4, 1962, nowabandoned, and assigned to the assignee of the present application, thehigh temperature of the glass provides several advantages in theelectrostatic deco rating process.

Further experience with production line methods and apparatus hasbrought to light several problems peculiar to the printing on surfacesat high temperatures.

In general, decorating processes of the type referred to above areperformed by interposing a stencil screen between a bed of printingpowder particles and the article surface to be decorated. An electricpower source is then connected across the powder supply and article tocharge the powder supply and article surface to electric potentials ofopposite polarity. In the usual case, the screen is at an intermediatepotential, usually at electrical ground. The electrical connectionsapply a charge to the powder particles and also establish an electricfield extending from the supply to the article surface which impels andattracts powder particles from the supply to the article surface. Thescreen is provided with apertures of the desired shape of the image, andbecause the field confines the particles to substantially .straight linemotion, the shape of the apertures in the screen determines the shape ofthe image on the article.

The screen usually takes the form of a relatively fine esh steel wirescreen which is coated in areas other than the image aperture to fillthe mesh of the screen. Because the screen, supply and article surfaceare quite close to each other during the process, and the articlesurface may be at a temperature of 1000" or more, it is quite obviousthat the heat radiating from the article surface influences both thescreen and supply.

It was found that after several exposures of the screen to heatedarticles in rapid succession, the wire of the screen became heated to arelatively high temperature. This caused both buckling or expansion ofthe screen and also undue bridging or clogging of the screen mesh in theaperture openings. Clogging of the screen was caused by powder particlesadhering, agglomerating, and melting on the hot wires of the screen.

Further, it was found that the electrical transfer characteristics ofthe powder rapidly decreased with the number of successive operations.

It is a primary object of the present invention to provide processes andapparatus for electrically applying printing powder particles to thesurface of heated articles wherein adverse effects produced by the hightemperatures of the articles are minimized.

It is a further object of the invention to provide methods and apparatusfor achieving the fore-going object which further improve or enablevariation of electrical properties and characteristics upon which theparticle transfer is dependent.

Other objects and features of the invention will become apparent byreference to the following specification and to the drawing.

In the drawing:

The single figure is a schematic diagram of an exemplary process andapparatus embodying the invention.

In the drawing, there is disclosed an apparatus wherein a supply or bedof printing powder particles 10 is supported upon the surface of anelectrically conductive plate 12. Bed 1t) and plate 12 are physicallylocated at the interior of a chamber 14 which is defined by a suitableside wall construction designated generally 16. The upper wall ofchamber 14 is defined by a stencil screen 18 which rests upon or issecured to the top of the wall 16 and is provided with a series ofsuitably shaped image defining apertures as at 20.

Screen 16 takes the form of a relatively fine mesh steel wire screenwhich is normally coated with a suitable coating which completely fillsthe openings in the screen mesh. Image defining apertures 20* areprovided by removing the coating from selected areas above the screen.Suitable screens and methods for preparing such screens are disclosed inUnited States Patent No. 3,100,150.

The surface of an article to be decorated is illustrated schematicallyat 22 and suitable supporting structure such as a frame 24 is providedso that the article surface can be located in the desired spacedregistry with screen 18 during the decorating operation.

Suitable sources of electric power schematically indicated at V1 and V2are electrically connected to plate 12, screen 18 and article 22, asillustrated in the drawing. Further details of suitable electricalconnections, etc., and potential magnitudes are set forth in detail in acopending application of William E. Johnson, Serial No. 393,817, filedAugust 31, 1964, and assigned to the assignee of this application.

A suitable blower 26 is connected to discharge gaseous fluid into theinterior of chamber 14 and is provided a diffuser 28 on its outlet, sothat gaseous fluid discharged into the interior of chamber 14 does notdisturb the powder in bed iii. The intake line 30 of blower 26 isconnected by branch lines 32 and 34 to a supply 36 of water saturatedair at ambient temperature and a waterfree liquid nitrogen supply 33from which cold nitrogen 3 vapor is withdrawn. Suitable regulatingvalves 40 and 42 are located in the branch conduits to proportion themixture withdrawn by blower 26.

Alternatively, supplies 36 and 38 may take the form of containers inwhich the gas or liquid is maintained under pressure. In this caseblower 26 would be replaced by a regulating or throttle valve.

It has been found, after experimentation, that best results are achievedby diffusing into chamber 14 a mixture of approximately equal volumes ofsaturated room temperature air and dry nitrogen vapor from a supply ofwater-free liquid nitrogen. The mixture is diffused into chamber 14 at arate such that a pressure differential of between one-half and fivepounds per square inch exists between the interior of chamber 14 and theambient air pressure outside. This pressure differential is suflicientto set up a gentle flow of gas through the image apertures of screen 18as indicated by arrows in the drawing, this flows of gaseous fluidfurther serving to assist in deflecting the radiant flow of heat fromthe article surface.

The pressure differential is selected in accordance with the mesh sizeof screen 18 and the area of the image apertures-Le. effectively thearea of the opening through the screen from chamber 14. For a #200 meshscreen with an image area of about 2 square inches, a pressuredifferential of 2 pounds per square inch is satisfactory.

Because of the relatively low temperature of the nitrogen vapor from theliquid nitrogen supply, a cooling action is achieved which minimizes theadverse effects of radiant heat from the article surface upon the wiremesh stencil screen, which may for convenience be termed a mechanicaleffect, as well as the effects of an undue amount of heat upon theelectrical characteristics of the printing process.

In a typical practice of a printing operation in accordance with theinvention, the article 22 which is being decorated might take the formof a flat sided glass flask or bottle which at the time of printingwould be at a temperature of approximately 1200" F. Bottles beingdecorated are held stationary in the position of article 22 in thedrawings for approximately 100 milliseconds and are advanced to thedecorating position at a rate of 40 bottles per minutei.e. one bottle isdecorated each one and one-half seconds. The total distance from thesurface being decoratedthe lower surface of article 22 and the uppersurface of powder bed 10 is one quarter of an inch, while the distancebetween the article surface being decorated and screen 18 isapproximately .007 inch. Screen 18 is a #200 mesh stainless steelscreen, prepared according to United States Patent 3,100,150.

A suitable glass frit composition for decorating hot glass bottles underthe above circumstances has the following composition of ingredients interms of percentage by weight.

Percent PbO 46 Pigment (selected from TiO cadmium selenide,

cobalt) 10 sio 25-32 B 3-5 R0 (this includes ZrO Fe O A1 0 BaO, ZnO,

CaO, NaO, and K 0 grouped as RO) 7-16 The glass frit described above isin the form of an extremely fine particle sized powder, the particlesize ranges preferably being between one and ten microns, althoughparticle sizes up to 50 microns can be successfully transferred underthe conditions described above.

In order for the particles to be transferred, it is necessary that theparticles be capable of being electrically charged. The abovecomposition is essentially of a nonconductive character and thepossibility of electrically charging the particles is primarilydependent upon the moisture content of the powder supply. The moisturecontent is most conveniently considered as a relatively thin layer ofmoisture on each of the individual particle s.

When the moisture content of the powder supply 18 within the range of.05 percent to .3 percent by weight, the individual particles can beelectrically charged and satisfactorily transferred from powder bed 10to the article surface. If the moisture content of the powder supplyfalls below .05 percent, the electrical characteristics of the powderbecome such that it is not possible to successfully transfer them, andat best an unsatisfactory printing of the article will result. If themoisture content of the supply exceeds .3 percent, the particles tend toagglomerate or stick to each other.

For reasons discussed in more detail in the copending Johnsonapplication Serial No. 393,817 it is desired that the electric fieldstrength between screen 18 and the adjacent surface of the powder bed berelatively high. With the above spacing of slightly less than one-fourthinch, the output of voltage source V1 is adjusted to give a potentialdifference of 8 to 10 kilovolts between screen 18 and the powder supply.

The upper field strengthi.e. the strength of the electric field betweenscreen 18 and powder bed support 12 is not as critical and the output ofvoltage V2 is selected to give a field strength of between 20 and 25volts per mil between screen 18 and the article surface.

With the foregoing physical arrangement, flat sided glass flasks are fedto the apparatus at the rate of one fiask per one and one-half seconddecorating cycle.

A mixture of equal volumes of air at percent relative humidity and atroom temperature and nitrogen at 0 percent relative humidity vapor froma supply of liquid nitrogen is continuously supplied to the apparatus ata rate sufficient to maintain a one pound per square inch pressuredifferential across screen 18. The composite relative humidity of theair-nitrogen mixture was between 45 and 50 percent and the temperatureof the mixture as it entered chamber 14 was 35 F.

Voltage sources V1 and V2 were pulsed or momentarily energized toestablish the electric fields referred to above during the millisecondinterval at which the flask was held stationary in the decoratingposition. Operation of sources V1 and V2 electrically charges and impelsparticles from supply bed 10 through the aperture of screen 18 to thearticle surface.

During operation under the above conditions, it was found that thescreen temperature was approximately 9 6 F. during the actual particletransfer cycle and rose to a maximum temperature of F. after theprinting. The maximum screen temperature was reached after the transferof the powder to the article surface occurred because the bottle orflask stayed in the decorating position, or close to it, for a shortperiod after the powder transfer had been completed.

Under the foregoing conditions, images of satisfactory density andresolution were achieved. Up to twelve consecutive prints at the abovedecorating rate can be achieved with satisfactory results. Thelimitation on the number of consecuive satisfactory prints is primarilydue to depletion and packing of the powder supply rather than adverseeffects created due to the 1200 F. flask temperature.

To compare the above results with results attainable in the absence of asupply of cooled humidified gas to chamber 14, the same physical set upwas employed on the same type flasks without cooling gas into chamber14. In this experiment, six bottles were fed through the apparatus atthe same rate of one bottle per one-half second interval as in theprevious case.

In this second experiment, the initial conditions within chamber 14found the chamber filled with air at atmospheric pressure at ambientroom temperature of about 80 F. and approximately 50 percent relativehumidity.

Of the group of six bottles which were fed through the apparatus duringthe second experiment, only the first bottle was printed. The screentemperature during the attempted printing of the sixth bottle was 415 F.Prior to the sixth attempted printing, the screen was at approximately380 F. and rose to a temperature of 420 F. before the bottle wasremoved. The screen Was found to be both blindedi.e., cloggedand warped.

The fact that only the first of the six bottles was printed during thelatter experiment was due to moisture loss in the frit occasioned by theexposure of the frit to the hot glass bottle. To maintain the .05percent to .3 percent by weight bulk moisture content of the powdersupply, the relative humidity in chamber 14 must be maintained atsomewhat above 20 percent. When chamber 14 is cyclically exposed to the1200 F. temperature of the bottles, unless some attempt is made to coolthe chamber and to continuously supply moisture to the chamber otreplace moisture evaporated by virtue of its exposure to the hot bottle,the frit powder becomes dried to a point where it can not beelectrically charged and transferred.

A second adverse electrical effect was encountered in the experimentwhere a cooling gas was not employed. This effect was that of thesubstantial decrease in the dielectric strength of the gaseous mediumwithin the electric field.

At temperatures reasonably close to normal room temperature, thedielectric strengthi.e., the electric field strength at which sparkingor breakdown occurs-is approximately 80 volts per mil for air. Thedielectric strength decreases substantially with increasing temperature,and at the 420 F. temperature which existed while the sixth bottle wasin decorating position, the dielectric strength of air decreases toapproximately 20 volts per mil. Thus, the heating effect produced byadvancing hot bottles in rapid succession into operative relationshipwith the decorating apparatus of the drawings will, in the absence ofthe supply of a cooled humidified gas to chamber 14, alter theelectrical characteristics of the particles to a point where they cannotbe successfully charged. The heating effect may also restrict theelectric potential difference, especially in the upper field, to a pointsuch that successful decoration could not be achieved even if it werepossible to charge the particles.

The dielectric strength of the lower field is also influenced by thehumidity of the incoming gas in that the potential difference across thefield increases when no charged particles are impelled from the bed toscreen 18. Movement of charged particles is analogous to a flow ofelectric current. In the event the moisture content of the powder bedbecomes such that no particles move from the bed to the screen the lowerfield loses the series resistance and the electric field strength isincreased, thereby increasing the tendency of the air or gaseous mediumto break down or spark.

The exemplary mixture of cold, dry nitrogen gas and substantiallysaturated air at room temperature is but one of many examples of acooled humidified gaseous mixture which would be operable under thespecific conditions outlined above. As a general rule, the gaseousmixture should be cool enough, and supplied at a rate such that underrepeated exposure of the apparatus to high temperature articles, thescreen temperature does not exceed room temperature by any substantialamount. Moisture should be supplied to chamber 14 by the gaseous fluidat a rate sufficient to maintain the moisture content of the powderwithin arange at which the powder can be electrically charged. Themoisture content of the powder necessary to enable the powder to beelectrically charged will vary in accordance with the composition of thepowder. However, for most glass frit powders the range of moisturecontent between .05 percent and .3 percent by weight bulk moisture isoperable.

Where the gaseous fluid takes the form of an airnitrogen mixture, themaintenance of a screen temperature of approximately F. assures that thedielectric strength of the gaseocs medium is sufficient to withstand theelectric fields to which it will be subjected. The constant supply of agaseous fluid to the apparatus also affords the possibility of employinggases having a higher dielectric strength than air where higher electricfield strengths may be required in the decorating process. Freon 12 andsulphur-hexafioride for example have dielectric strengths greater thantwice the dielectric strength of air.

It should also be noted that temperature has a substantial effect on thedielectric strength of a gas and that vapor from liquid nitrogen is atleast theoretically available at minus 320 F. In cases where anextremely high dielectric strength is required, the temperature andcomposition of the gaseous fluid employed may be chosen accordingly.

The cool gaseous fluid does not have any practical temperature effectupon the bottle being decorated because of the extremely hightemperature, the relatively large mass of the hot glass, and therelatively short time which each individual bottle is exposed to theapparatus. The apparatus, on the other hand, is exposed to a rapidsuccession of relatively massive bottles at extremely high temperatures.

While one embodiment of the invention has been described in detail, itwill be apparent to those skilled in the art that the disclosedembodiment may be modified. Therefore, the foregoing description is tobe considered exemplary rather than limiting and the true scope of theinvention is that defined in the following claims.

I claim:

1. The method of applying an image shaped layer of powdered fritparticles to an article surface heated to a relatively high temperaturecomprising the steps of interposing a stencil screen having an imagedefining aperture therethrough between a supply of frit particlescapable of being electrically charged and the surface of a heatedarticle to be decorated with the screen disposed in spaced relationshipto the supply and to the article surface, diffusing into the spacebetween said supply and said screen a moisture laden gaseous fluid at atemperature substantially less than that of the article surface, andestablishing an electric field between said supply and article surfaceoperable to electrically charge and impel particles from said supplythrough the apertures of said screen to said article surface.

2. The method as defined in claim 1 further comprising the steps ofmaintaining a continuous flow of said gaseous fluid from said spacebetween said supply and said screen through the image apertures of saidscreen to constitute said gaseous fluid as the dielectric mediumsubstantially throughout the space between said supply and said articlesurface.

3. The method of applying an image shaped layer of frit particles to aglass article surface heated to a temperature of the order of 1000 F.comprising the steps of interposing a stencil screen having an imagedefining aperture therethrough between a supply of frit particlescapable of being electrically charged and the surface of a heatedarticle to be decorated with the screen disposed in spaced relationshipto the supply and to the article surface, enclosing the supply to definea chamber containing said supply having a wall defined by said screen,diffusing into said chamber a moisture laden gaseous fluid cooled to atemperature substantially below room temperature to cool said chamberand said screen, and establishing an electric field between said supplyand article surface of a field strength sufficient to electricallycharge and impel particles from said supply through the apertures ofsaid screen to said article surface and of a strength less than thedielectric strength of said gaseous fluid.

4. The method as defined in claim 3 further comprising the steps ofcontrolling the moisture content of the gaseous fluid to maintain themoisture content of the powder supply substantially constant in the faceof the exposure of said supply to heat radiated from the article.

5. The method as defined in claim 3 further comprising the step ofdiffusing said gaseous fluid into said chamber at a rate suflicient toestablish a continuous flow of said fluid from said chamber through theimage aperture of said screen into the space between said screen andarticle surface whereby said gaseous fluid constitutes the dielectricmedium of said electric field.

6. The method of applying an image shaped layer of powdered fritparticles to a glass article surface heated to a temperature of theorder of 1000 F. comprising the steps of locating a supply of fritparticles capable of being electrically charged in a chamber having itstop defined by a stencil screen having an image defining aperturetherethrough, positioning the surface of a heated article to bedecorated in vertically spaced registry above the screen, diffusing intosaid chamber a cooled humidified gaseous fluid consisting of a mixtureof approximately equal volumes of vapor from a liquefied gas andsaturated air at ambient temperature, and establishing an electric fieldbetween said supply and article surface operable to electrically chargeand impel particles from said supply through the apertures of saidscreen to said article surface.

7. The method as defined in claim 6 wherein said gaseous fluid isdiffused into said chamber at a rate to establish and maintain apressure differential of between approximately /2 p.s.i. and 5 p.s.i.between the interior and exterior of said chamber to thereby establishand maintain a continuous flow of said gaseous fluid through the imageapertures of said screen into the space between the screen and articlesurface.

8. Apparatus for applying an image shaped layer of powder particles tothe surface of each of a plurality of articles while said articles areheated to a relatively high temperature comprising means defining achamber having one wall thereof defined by a stencil screen having imagedefining apertures therethrough, means for supporting a supply ofprinting powder particles within said chamber in spaced registry withthe image defining apertures of said screen, support means forsupporting the surface of a heated article to be decorated in adjacentspaced relationship to said screen at the exterior of said chamber,electric power supply means for establishing an electric field betweenthe supply and article surface operable to charge the powder particlesof said supply and impel the charged particles through the imageapertures to the article surface while the article is supported uponsaid support means, and means for maintaining the 8 moisture content ofsaid supply of printing powder particles substantially constant duringexposure of said supply to successive heated articles supported uponsaid support means.

9. Apparatus as defined in claim 8 wherein the means for maintaining themoisture content include means for supplying to said chamber anadjustably regulated mixture of a plurality of gases in knownproportions at a temperature substantially below that of the heatedarticles and of controlled humidity.

10. The method of decorating glass articles heated to temperatures ofthe order of 1000 F. by applying an image shaped layer of powdered fritparticles to a surface of each article comprising the steps ofsupporting in a bed a supply of frit particles having a moisture contentsufiicient to enable the particles to be electrically charged upon anelectrically conductive plate in spaced registry beneath a stencilscreen having an image defining aperture therethrough, advancing heatedglass articles in succession to a decorating position in closely spacedregistry above said stencil screen, applying electric voltage pulses ofopposite polarity to said plate and each article while the article is insaid decorating position to electrically charge and impel particles fromsaid bed through the aperture of said screen to the registered articlesurface, and continuously diffusing a cooled humidified gas into thespace between said screen and said bed to maintain the moisture contentof said particles and the temperature of said screen substantiallyconstant in the face of repeated exposure to the heated glass articles.

11. The method as defined in claim 10 further comprising the steps ofadjusting the moisture content of the supply of particles to an initialmoisture content of between .05 percent and .3 percent by weight, andregulating the moisture content of said gas to maintain the relativehumidity of the gas between said bed and said screen at at least 20percent during the decorating of the articles.

References Cited by the Examiner UNITED STATES PATENTS 2,162,317 6/1939ReZ.

2,842,456 7/1958 Carlson 117-175 X 2,861,543 11/1958 Grumrine et al.117-17.5 X 2,873,721 2/1959 Andrus et al. 117-175 X 2,940,864 6/1960Watson.

3,005,726 10/1961 Olson 11717.5 X

FOREIGN PATENTS 81,920 9/1956 Denmark.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,294,017 December 27, 1966 Douglas P. St. John It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 4, line 70, for "one-half" read one and one-half column 5, line19, for "0t" read to column 6, line 4, for "gaseocs" read gaseous Signedand sealed this 24th day of October 1 967 (SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

1. THE METHOD OF APPLYING AN IMAGE SHAPED LAYER OF POWDERED FRITPARTICLES TO AN ARTICLE SURFACE HEATED TO A RELATIVELY HIGH TEMPERATURECOMPRISING THE STEPS OF INTERPOSING A STENCIL SCREEN HAVING AN IMAGEDEFINING APERTURE THERETHROUGH BETWEEN A SUPPLY OF FRIT PARTICLESCAPABLE OF BEING ELECTRICALLY CHARGED AND THE SURFACE OF A HEATEDARTICLE TO BE DECORATED WITH THE SCREEN DISPOSED IN SPACED RELATIONSHIPTO THE SUPPLY AND TO THE ARTICLE SURFACE, DIFFUSING INTO THE SPACEBETWEEN SAID SUPPLY AND SAID SCREEN A MOISTURE LADEN GASEOUS FLUID AT ATEMPERATURE SUBSTANTIALLY LESS THAN THAT OF THE ARTICLE SURFACE, ANDESTABLISHING AN ELECTRIC FIELD BETWEEN SAID SUPPLY AND ARTICLE SURFACEOPERABLE TO ELECTRICALLY CHARGE AND IMPEL PARTICLES FROM SAID SUPPLYTHROUGH THE APERTURES OF SAID SCREEN TO SAID ARTICLE SURFACE.